There exist systems for wirelessly transmitting power (i.e., energy). In Japanese Unexamined Patent Application Publication No. 2002-101578, electromagnetic induction techniques are discussed. The above system includes an electromagnetic wave technique. In Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-501510, magnetic resonance techniques are discussed. In all of the above techniques, various contrivances increase the power supply efficiency. Japanese Unexamined Patent Application Publication No. 2002-101578 discloses technology for a contactless power transfer apparatus that wireless transmits power by electromagnetic induction. In this apparatus, when the relative positions of a primary coil (i.e., the transmitting coil) and a secondary coil (i.e., the receiving coil) change, factors such as the frequency of the current flowing in the primary coil are adjusted to keep the output from the secondary coil constant.